Thomas Gnauk

Particle number size distributions in a street canyon and their transformation into the urban-air background: measurements and a simple model study

Abstract Car traffic is one of the main anthropogenic aerosol sources in modern cities. The characterization of these emissions is important for describing the quality of urban air. Measurements in a street canyon in a German urban area were made. Maximum number concentrations occurred during morning hours from Monday to Friday when the traffic density is highest. The maximum of the number size distribution measured during rush hour near a busy city street was at a particle diameter of 15 nm . This differs significantly from size distributions directly measured in vehicle exhaust (vehicles placed on chassis dynamometers used for vehicle emissions certification), typically about 50 nm . The size distributions measured in the urban area depended on the distance to the nearest road. With increasing distance, the maximum of the size distribution increased, and the total number concentration decreased. This seems to be a result of particle growth due to processes such as coagulation and condensation, and dilution with the surrounding air. To clarify the transformation of the particle number size distributions measured in a street canyon into the urban-air background, a sectional aerosol model was used to calculate the evolution of the number size distribution, and included the effect of condensation, coagulation, dilution, and continuous entrainment of freshly emitted particles yielding good agreement with measurements.

Enhanced Role of Transition Metal Ion Catalysis During In-Cloud Oxidation of SO2

Dust in the Clouds Sulfate aerosols have the greatest radiative impact on climate systems. Harris et al. (p. 727) report that the oxidation of sulfur dioxide gas, catalyzed by natural transition metal ions mostly on the surface of coarse mineral dust, is the dominant pathway for sulfate production in clouds. In view of the growing sulfur dioxide emissions from large, industrializing countries, including this process in climate models should improve the agreement between models and observations. Transition metal ions catalyze most of the oxidation of sulfur dioxide that occurs in clouds. Global sulfate production plays a key role in aerosol radiative forcing; more than half of this production occurs in clouds. We found that sulfur dioxide oxidation catalyzed by natural transition metal ions is the dominant in-cloud oxidation pathway. The pathway was observed to occur primarily on coarse mineral dust, so the sulfate produced will have a short lifetime and little direct or indirect climatic effect. Taking this into account will lead to large changes in estimates of the magnitude and spatial distribution of aerosol forcing. Therefore, this oxidation pathway—which is currently included in only one of the 12 major global climate models—will have a significant impact on assessments of current and future climate.

Mixing state of elemental carbon and non-light-absorbing aerosol components derived from in situ particle optical properties at Xinken in Pearl River Delta of China

Received 29 November 2005; revised 28 April 2006; accepted 5 June 2006; published 21 October 2006. [1] The aerosol mixing state was investigated with an optical closure study at Xinken, Pearl River Delta of China in 2004. On the basis of in situ aerosol microphysical and chemical measurements and a two-component aerosol optical model an internal consistency algorithm was developed to model the mass ratio (r) of externally mixed elemental carbon (EC) to total EC, which minimized the discrepancies between measured and calculated optical properties. The rest of EC was assumed to be internally mixed. A time series of r was retrieved. Good agreement between model and observation was found, on the order of ±15% for total/back scattering coefficients and ±10% for absorption coefficient. The EC mixing state was strongly dependent on the local wind patterns. When north/northeasterly winds prevailed, the air came from the urban and industrial areas of mainland China, and EC was mainly externally mixed with an average r of 85 ± 12%. When the airflow was controlled by a weak local wind system, the mixing state showed a pronounced diurnal variation. During daytime the wind speed was nearly zero. This favored the increase of local pollution, and the average r was about 95%. However, during nighttime the EC mixing state transformed to be internally mixed apparently with an average r of 53 ± 15%, which can be explained by a more aged air mass. The south/ southeasterlywindscomingfromtheseawerefoundtohavethemostimportanteffectonthe transformation of EC mixing state in the night, but fairly rapid local aging processing was also observed. The uncertainties of the model were explored by a Monte Carlo simulation.

Performance of an Aerodyne Aerosol Mass Spectrometer (AMS) during Intensive Campaigns in China in the Summer of 2006

An Aerodyne quadrupole aerosol mass spectrometer (AMS) was deployed in China in the summer of 2006. The measurements were made in the Pearl River Delta region in July 2006 (PRD campaign) and also in Beijing in August–September 2006 (CAREBEIJING campaign). The AMS successfully measured size-resolved chemical composition of submicron non-refractory aerosol (vaporized at 600°C in vacuum) with a time resolution of 10 min, although some quantification issues have been identified. We observed extremely large signals at m/z 39 ( 39 K + ) and 41 (41K + ), which significantly exceeded m/z 28 (N + 2 ) signals. We also found large signals of m/z 85 ( 85 Rb + ), 87 (87Rb + ), and 133 (Cs + ). Laboratory experiments suggest that the large enhancement of K + could have been due to the presence of K-containing particles in ambient air. The interferences of alkali metals at m/z 41, 85, 87, and 133 were significant and need to be corrected for better quantification of organic aerosol. The AMS measurements are compared with other, collocated measurements: a particle-into-liquid sampler combined with an ion chromatograph (PILS-IC), a Sunset Laboratory semi-continuous carbonaceous aerosol analyzer, and a Berner impactor sampler followed by off-line ion chromatography analysis (for major inorganic ions). We have found good agreement between the AMS and the other instruments when we assume an AMS particle collection efficiency (CE) of 0.5 for the PRD data and CE = 1.0 for the CAREBEIJING data. These results suggest that the AMS CE could be significantly different in different locations. Possible factors affecting the variability in the CE values are discussed.

Laboratory studies on secondary organic aerosol formation from terpenes

The formation of secondary organic aerosol (SOA) following the ozonolysis of terpene has been investigated intensively in recent years. The enhancement of SOA yields from the acid catalysed reactions of organics on aerosol surfaces or in the bulk particle phase has been receiving great attention. Recent studies show that the presence of acidic seed particles increases the SOA yield significantly (M. S. Jang and R. M. Kamens, Environ. Sci. Technol., 2001, 35, 4758, ref. 1; M. S. Jang, N. M. Czoschke, S. Lee and R. M. Kamens, Science, 2002, 298, 814, ref. 2; N. M. Czoschke, M. Jang and R. M. Kamens, Atmos. Environ., 2003, 37, 4287, ref. 3; M. S. Jang, B. Carroll, B. Chandramouli and R. M. Kamens, Environ. Sci. Technol., 2003, 37, 3828, ref. 4; Y. Iinuma, O. Böge, T. Gnauk and H. Herrmann, Atmos. Environ., 2004, 38, 761, ref. 5; S. Gao, M. Keywood, N. L. Ng, J. Surratt, V. Varutbangkul, R. Bahreini, R. C. Flagan and J. H. Seinfeld, J. Phys. Chem. A, 2004, 108, 10147, ref. 6). More detailed studies report the formation of higher molecular weight products in SOA (refs. 5 and 6; M. P. Tolocka, M. Jang, J. M. Ginter, F. J. Cox, R. M. Kamens and M. V. Johnston, Environ. Sci. Technol., 2004, 38, 1428, ref. 7; S. Gao, N. L. Ng, M. Keywood, V. Varutbangkul, R. Bahreini, A. Nenes, J. He, K. Y. Yoo, J. L. Beauchamp, R. P. Hodyss, R. C. Flagan and J. H. Seinfeld, Environ. Sci. Technol., 2004, 38, 6582, ref. 8) which could result in a non-reversible uptake of organics into the particle phase. Most of the past studies concentrated on the characterisation of the yields of enhanced SOA and its composition from ozonolysis of terpenes in the presence or absence of acidic and neutral seed particles. Recent findings from cyclohexene ozonolysis show that the presence of OH scavengers can also significantly influence the SOA yield. Our new results from the IfT chemistry department aerosol chamber on terpene ozonolysis in the presence of OH scavengers show that the presence of hydroxyl radical scavengers clearly reduces the amount of formed SOA. The OH scavenger strongly depletes the formation of oligomeric compounds in the particle phase in contrast to previous findings (M. D. Keywood, J. H. Kroll, V. Varatbangkul, R. Bahreini, R. C. Flagan and J. H. Seinfeld, Environ. Sci. Technol., 2004, 38, 3343, ref. 9). This result indicates that hydroxyl radicals play an important role in the formation of precursor compounds (e.g., hydroxy pinonaldehyde) for the particle phase heterogeneous acid catalysed reactions leading to the higher molecular weight compounds and thus the enhancement of SOA yields. Better understanding of the role of hydroxyl radicals in the formation of SOA is necessary to distinguish between the contribution of ozonolysis and hydroxyl radicals to the SOA yield. If the recent findings are a ubiquitous phenomenon in the atmosphere, current atmospheric and climate models might underestimate SOA formation yields, particle phase OC contents and its impact on the atmospheric radiation budget.

Monoterpene emissions and carbonyl compound air concentrations during the blooming period of rape (Brassica napus).

An increasing percentage of agricultural land in Germany is used for oil seed plants. Hence, rape has become an important agricultural plant (in Saxony 1998: 12% of the farmland) in the recent years. During flowering of rape along with intensive radiation and high temperatures, a higher production and emission of biogenic VOC was observed. The emissions of terpenes were determined and more importantly, high concentrations of organic carbonyl compounds were observed during this field experiment. All measurements of interest have been carried out during two selected days with optimal weather conditions. It is found that the origin or the mechanism of formation of different group of compounds had strong influence on the day to day variation of their concentrations. The emission flux of terpenes from flowering rape plants was determined to be 16-32 microg h(-1) m(-2) (30-60 ng h(-1) per g dry plant-540-11080 ng h(-1) per plant), in total. Limonene, alpha-thujene and sabinene were the most important compounds (about 60% of total terpenes). For limonene and sabinene reference emission rates (Ms) and temperature coefficients were determined: beta(limonene) = 0.108 K(-1) and Ms = 14.57 microg h(-1) m(-2) beta(sabinene) = 0.095 K(-1) and Ms = 5.39 microg h(-1) m(-2). The detected carbonyl compound concentrations were unexpectedly high (maximum formaldehyde concentration was 18.1 ppbv and 3.4 ppbv for butyraldehyde) for an open field. Possible reasons for these concentrations are the combination of primary emission from the plants induced by high temperature and high ozone stress, the secondary formation from biogenically and advected anthropogenically emitted VOC at high radiation intensities and furthered by the low wind speeds at this time.

A new method to study aerosol source contributions along the tracks of air parcels and its application to the near-ground level aerosol chemical composition in central Europe

Abstract A novel method is presented to reveal the significance and contribution of source types and characteristic formation times for individual aerosol constituents: Backward trajectory analyses are used to allocate time-resolved information about residence time of air masses over different types of ground surfaces. The correlations between the residence time of air mass over individual ground surface types and aerosol constituent concentrations (or particulate matter mass fractions) are investigated by a time-weighting method. The correlation coefficients between the concentrations of individual aerosol constituents and the residence times of air masses over certain types of ground surfaces at a certain time difference to arrival time were used to compose time profiles. These are suggested to reflect the time-resolved ground emissions’ influence on aerosol composition, which is particularly relevant for secondary aerosol constituents. The method has been applied to aerosol chemical composition data from various seasons and from rural and urban sites in Germany. For various ground types, we obtain correlations between weighted (and normalized) residence times (‘source loadings’) on one hand and the abundances of trace constituents known as markers for marine (Na, Cl), continental-rural (e.g. mineral dust components) and industrial sources (e.g., organic and elemental C, As, Pb) on the other hand. The occurrence of super-μm particulate NO 3 − in central Europe is found to originate largely in the marginal seas. The time profiles indicate that the characteristic formation time of the secondary aerosol is 48– 72 h , while the coarse mode particulate matter including some heavy metals was determined by emissions h back. The occurrence of particulate elemental carbon was temporally bimodal with regard to the elapsed time since emission (maxima at Δ t≈60 h and Δ t =12– 24 h ), which indicates the presence of two types undergoing a selection process during aging. The factors which explained most of the variability of the aerosol chemical composition were the season and the type of ground surface in contact with the air mass during its transport. More immediate influences on the samples, such as the weather conditions during sampling and the type of site (rural or urban) were distinctly less significant.

Diurnal Variations of Atmospheric Hydrogen Peroxide Concentrations in Saxony (Germany)

Abstract During a 3-year study, gaseous hydrogenperoxide (H2O2) concentrations were measuredas part of the SANA project at the Melpitz FieldResearch Station and in the city of Leipzig. Typicaldaily mean H2O2 mixing ratios on sunny dayswere 0.15 to 0.25 ppbv with maximum values of 0.3 to0.5 ppbv at Melpitz, and 0.3 to 0.6 ppbv with maximumvalues of 0.4 to 1.0 ppbv in Leipzig. Over the entireperiod of the project the maximum hourly mean valueswere 2.1 ppbv and 5.3 ppbv in Melpitz and Leipzig,respectively. The data were not complete enough to show a trend.Linear regression analysis shows, that ozone(O3), temperature and solar radiation arepositively correlated with H2O2, whereasnitrogen oxides (NOx), carbon monoxide (CO) andrelative humidity are negatively correlated. Negativecorrelation between H2O2 and CO is caused byjoint occurrence of CO with NOx in exhaust gases.Negative correlation between H2O2 andrelative humidity is not necessarily in contradictionto the accelerating effect of water vapour onH2O2 formation. The strong positivecorrelation of H2O2 with the dew pointdifference however seems to better reflect theinfluence of water vapour. Multiple linear regression analysis (MLRA) of thecomponents measured, indicates the great influence of CO on the formation of H2O2 in the gasphase.

Size-resolved aerosol characterization for a polluted episode at the IfT research station Melpitz in autumn 1997

Inorganic ions, organic carbon (OC), elemental carbon (EC) and a variety of organic single species in airborne particles have been determined at the research station of the Leibniz – Institut für Troposphärenforschung (IfT) in Melpitz (Germany) in autumn 1997.Samples of eight selected measurement events were divided in two groups in order to investigate differences in the chemical composition of particles originating from southwesterly (SW – developed EU countries) or from easterly directions (E – less developed eastern countries). Differences between these two groups were tested statistically by Students t-test.Five stage cascade impactor samples show nitrate as most abundant in the accumulation mode in the SW group. EC and sulphate show the most abundant mass fractions in the E group. That can be considered as a consequence of domestic coal heating and coal-fired power plant emissions in the region of westerly Poland, northern Czech Republic and easterly Germany. Higher nitrate concentrations in the SW group can be explained by stronger NOx emissions caused by the leeward plume of the conurbation of Leipzig, as well as by the still higher traffic density in western Germany.The methane sulphonic acid (MSA) mass fraction was higher for SW air masses in accumulation mode particles, probably indicating marine origin. Succinic acid also showed higher mass fractions for the SW group. This could be caused by primary emission in automobile exhaust gases and photochemical formation during transport from SW. Indeed, during SW sampling, solar radiation intensity was higher than during E sampling.The observed differences in the particle composition are an expression of the still existing technology gradient in Europe. Future campaigns could show the development to a joint economy with smaller differences in anthropogenic emissions.

A Three-year Study of Nonmethane Hydrocarbons in Surface Air over Saxony (Germany)

Measurements of the nonmethane hydrocarbon (NMHC) mixing ratio over a period of 42 months were carried out for the first time in the air of a research station situated in the former East Germany during the SANA project. Apart from four species, all other species analysed showed a statistically significant downward trend at the 95% significance level. The decrease of the hydrocarbon concentrations was superimposed by seasonal variations. A drop of about 40% of the annual mean values from 1993 to 1995 was observed. This development reflects the dramatic changes in traffic, industry, power economy, and agriculture in Saxony after the reunification of Germany. The remove of two-stroke engined cars is reflected in NMHC mixing ratio changes, as is the removal of obsolete chemical plants. Generally it was not possible to relate causes and effects of a single event, but in some cases major changes in concentrations and NMHC ratios occurred coincidentally with the disappearance of a specific emission source.